The present invention relates to the catalyzation of methylene dianiline complexes utilized in the curing of amine curable polymers and particularly polyurethane polymers so that cure proceeds rapidly and at reasonably low temperatures.
Heretofore, the curing of amine curable polymers or prepolymers such as epoxy resins, millable halogen containing hydrocarbon polymers and particularly isocyanate terminated polyurethane prepolymers have generally involved the blending of an amine curing agent, forming of the resulting mixture into a useful form and heating to complete the curing reaction. However, a problem generally encountered by this procedure was the premature reaction of the curing agent with the curable polymer during the mixing and formation operations. This problem was particularly acute in highly reactive systems such as in the curing of isocyanate terminated polyurethane prepolymers which necessitated the use of special low residence time mixers and selected diamine curing agents having reduced reactivity which generally limited the physical properties of the vulcanizates.
A class of widely used curing agents which have overcome many of the above-noted problems are the complexes of methylene dianiline and a salt which upon heating generally in excess of temperatures of 100.degree. C. liberated the methylene dianiline from the complex and allowed it to commence curing of the polymers (U.S. Pat. No. 3,755,261). Although an improvement in the art, such class of curing agents still tended to be time consuming, required undesirable demolding time and thus tended to be uneconomical due to the number of molds required. The only way to overcome these disadvantages was to increase the cure temperature which resulted, of course, in an increased cure rate. However, increased cure temperatures caused larger thermal expansion and subsequent shrinkage in the molded article which often led to strains and cracks in the product.